Self-assembly of a robust hydrogen-bonded octylphosphonate network on cesium lead bromide perovskite nanocrystals for light-emitting diodes.

We report the self-assembly of an extensive inter-ligand hydrogen-bonding network of octylphosphonates on the surface of cesium lead bromide nanocrystals (CsPbBr3 NCs). The post-synthetic addition of octylphosphonic acid to oleic acid/oleylamine-capped CsPbBr3 NCs promoted the attachment of octylphosphonate to the NC surface, while the remaining oleylammonium ligands maintained the high dispersability of the NCs in non-polar solvent. Through powerful 2D solid-state 31P-1H NMR, we demonstrated that an ethyl acetate/acetonitrile purification regime was crucial for initiating the self-assembly of extensive octylphosphonate chains. Octylphosphonate ligands were found to preferentially bind in a monodentate mode through P-O-, leaving polar P[double bond, length as m-dash]O and P-OH groups free to form inter-ligand hydrogen bonds. The octylphosphonate ligand network strongly passivated the nanocrystal surface, yielding a fully-purified CsPbBr3 NC ink with PLQY of 62%, over 3 times higher than untreated NCs. We translated this to LED devices, achieving maximum external quantum efficiency and luminance of 7.74% and 1022 cd m-2 with OPA treatment, as opposed to 3.59% and 229 cd m-2 for untreated CsPbBr3 NCs. This represents one of the highest efficiency LEDs obtained for all-inorganic CsPbBr3 NCs, accomplished through simple, effective passivation and purification processes. The robust binding of octylphosphonates to the perovskite lattice, and specifically their ability to interlink through hydrogen bonding, offers a promising passivation approach which could potentially be beneficial across a breadth of halide perovskite optoelectronic applications.

Low threshold and efficient multiple exciton generation in halide perovskite nanocrystals.

Multiple exciton generation (MEG) or carrier multiplication, a process that spawns two or more electron-hole pairs from an absorbed high-energy photon (larger than two times bandgap energy Eg), is a promising way to augment the photocurrent and overcome the Shockley-Queisser limit. Conventional semiconductor nanocrystals, the forerunners, face severe challenges from fast hot-carrier cooling. Perovskite nanocrystals possess an intrinsic phonon bottleneck that prolongs slow hot-carrier cooling, transcending these limitations. Herein, we demonstrate enhanced MEG with 2.25Eg threshold and 75% slope efficiency in intermediate-confined colloidal formamidinium lead iodide nanocrystals, surpassing those in strongly confined lead sulfide or lead selenide incumbents. Efficient MEG occurs via inverse Auger process within 90 fs, afforded by the slow cooling of energetic hot carriers. These nanocrystals circumvent the conundrum over enhanced Coulombic coupling and reduced density of states in strongly confined nanocrystals. These insights may lead to the realization of next generation of solar cells and efficient optoelectronic devices.

Following the Kinetics of Barium Titanate Nanocrystal Formation in Benzyl Alcohol Under Near-Ambient Conditions.

In complex chemical syntheses (e.g., coprecipitation reactions), nucleation, growth, and coarsening often occur concurrently, obscuring the individual processes. Improved knowledge of these processes will help to better understand and optimize the reaction protocol. Here, a form-free and model independent approach, based on a combination of time-resolved small/wide-angle X-ray scattering, is employed to elucidate the effect of reaction parameters (such as precursor concentration, reactant stoichiometry, and temperature) on the nucleation, crystallization, and growth phenomena during the formation of nanocrystalline barium titanate. The strength of this approach is that it relies solely on the total scattered intensity (i.e., scattering invariant) of the investigated system, and no prior knowledge is required. As such, it can be widely applied to other synthesis protocols and material's systems. Through the scattering invariant, it is found that the amorphous-to-crystalline transformation of barium titanate is predominantly determined by the total amount of water released from the gel-like barium hydroxide octahydrate precursor, and three rate-limiting regimes are established. As a result of this improved understanding of the effect of varying reaction conditions, elementary boundary conditions can be set up for a better control of the barium titanate nanocrystal synthesis.

Precursor non-stoichiometry to enable improved CH3NH3PbBr3 nanocrystal LED performance.

High photoluminescence quantum yields and narrow emission wavelengths, combined with low temperature solution processing, make CH3NH3PbBr3 nanocrystals (NCs) favorable candidates for light-emitting applications. Herein, we describe the synthesis of CH3NH3PbBr3 NC inks by a convenient room-temperature ligand assisted reprecipitation protocol. We further investigate the effect of modulation of the CH3NH3Br : PbBr2 ratio during NC synthesis on the optical properties, crystallinity, particle size distribution and film formation of the NC ink. Subsequently, we fabricate LEDs using these NCs as the emissive layer and the highest efficiency (1.75% external quantum efficiency) and brightness (>2700 cd m-2) is achieved for the 1.15 : 1 precursor ratio. It is inferred that the NC surface properties and film coverage are more crucial than the photoluminescence intensity to achieve high device efficiency. Moreover, by separating the NC synthesis and thin film formation processes, we can exert more control during device fabrication, which makes it very promising for scale-up applications.

Inducing Isotropic Growth in Multidimensional Cesium Lead Halide Perovskite Nanocrystals.

A new two-step synthetic protocol to yield monodisperse spherical zero-dimensional (0D) Cs4 PbX6 nanocrystals (NCs) and three-dimensional (3D) CsPbX3 NCs is described. The first step of the reaction involves the colloidal synthesis of spherical PbX2 seed NCs, which are subsequently converted to Cs4 PbX6 and CsPbX3 NCs through hot injection of a Cs precursor at the desired reaction temperatures. By employing less reactive Pb and halide precursors, the reaction time was extended from several seconds to about five minutes, thereby allowing greater control during the crystallization and growth stages. The adjustment of halide ratios allows color tuning over a wide spectral range (411-669 nm) for CsPbX3 NCs, with high photoluminescence quantum yields (6-65 %) and narrow emission line widths (ca. 13-30 nm). We envisage our spherical NCs to become a starting point for shell growth (e.g., ZnS, CdS, PbS) by overcoming the difficulty of shell growth around thermodynamically unfavorable (i.e., high surface free energy) cuboid-shaped NCs.

Benzyl Alcohol-Treated CH3NH3PbBr3 Nanocrystals Exhibiting High Luminescence, Stability, and Ultralow Amplified Spontaneous Emission Thresholds.

We report the high yield synthesis of about 11 nm sized CH3NH3PbBr3 nanocrystals with near-unity photoluminescence quantum yield. The nanocrystals are formed in the presence of surface-binding ligands through their direct precipitation in a benzyl alcohol/toluene phase. The benzyl alcohol plays a pivotal role in steering the surface ligands binding motifs on the NC surface, resulting in enhanced surface-trap passivation and near-unity PLQY values. We further demonstrate that thin films from purified CH3NH3PbBr3 nanocrystals are stable >4 months in air, exhibit high optical gain (about 520 cm-1), and display stable, ultralow amplified spontaneous emission thresholds of 13.9 ± 1.3 and 569.7 ± 6 µJ cm-2 at one-photon (400 nm) and two-photon (800 nm) absorption, respectively. To the best of our knowledge, the latter signifies a 5-fold reduction of the lowest reported threshold value for halide perovskite nanocrystals to date, which makes them ideal candidates for light-emitting and low-threshold lasing applications.

Giant five-photon absorption from multidimensional core-shell halide perovskite colloidal nanocrystals.

Multiphoton absorption processes enable many technologically important applications, such as in vivo imaging, photodynamic therapy and optical limiting, and so on. Specifically, higher-order nonlinear absorption such as five-photon absorption offers significant advantages of greater spatial confinement, increased penetration depth, reduced autofluorescence, enhanced sensitivity and improved resolution over lower orders in bioimaging. Organic chromophores and conventional semiconductor nanocrystals are leaders in two-/three-photon absorption applications, but face considerable challenges from their small five-photon action cross-sections. Herein, we reveal that the family of halide perovskite colloidal nanocrystals transcend these constraints with highly efficient five-photon-excited upconversion fluorescence-unprecedented for semiconductor nanocrystals. Amazingly, their multidimensional type I (both conduction and valence band edges of core lie within bandgap of shell) core-shell (three-dimensional methylammonium lead bromide/two-dimensional octylammonium lead bromide) perovskite nanocrystals exhibit five-photon action cross-sections that are at least 9 orders larger than state-of-the-art specially designed organic molecules. Importantly, this family of halide perovskite nanocrystals may enable fresh approaches for next-generation multiphoton imaging applications.

Atomically Altered Hematite for Highly Efficient Perovskite Tandem Water-Splitting Devices.

Photoelectrochemical (PEC) cells are attractive for storing solar energy in chemical bonds through cleaving of water into oxygen and hydrogen. Although hematite (α-Fe2 O3 ) is a promising photoanode material owing to its chemical stability, suitable band gap, low cost, and environmental friendliness, its performance is limited by short carrier lifetimes, poor conductivity, and sluggish kinetics leading to low (solar-to-hydrogen) STH efficiency. Herein, we combine solution-based hydrothermal growth and a post-growth surface exposure through atomic layer deposition (ALD) to show a dramatic enhancement of the efficiency for water photolysis. These modified photoanodes show a high photocurrent of 3.12 mA cm-2 at 1.23 V versus RHE, (>5 times higher than Fe2 O3 ) and a plateau photocurrent of 4.5 mA cm-2 at 1.5 V versus RHE. We demonstrate that these photoanodes in tandem with a CH3 NH3 PbI3 perovskite solar cell achieves overall unassisted water splitting with an STH conversion efficiency of 3.4 %, constituting a new benchmark for hematite-based tandem systems.

Influence of Solution Properties and Process Parameters on the Formation and Morphology of YSZ and NiO Ceramic Nanofibers by Electrospinning.

The fabrication process of ceramic yttria-stabilized zirconia (YSZ) and nickel oxide nanofibers by electrospinning is reported. The preparation of hollow YSZ nanofibers and aligned nanofiber arrays is also demonstrated. The influence of the process parameters of the electrospinning process, the physicochemical properties of the spinning solutions, and the thermal treatment procedure on spinnability and final microstructure of the ceramic fibers was determined. The fiber diameter can be varied from hundreds of nanometers to more than a micrometer by controlling the solution properties of the electrospinning process, while the grain size and surface roughness of the resulting fibers are mainly controlled via the final thermal annealing process. Although most observed phenomena are in qualitative agreement with previous studies on the electrospinning of polymeric nanofibers, one of the main differences is the high ionic strength of ceramic precursor solutions, which may hamper the spinnability. A strategy to control the effective ionic strength of precursor solutions is also presented.

Flexible Yttrium-Stabilized Zirconia Nanofibers Offer Bioactive Cues for Osteogenic Differentiation of Human Mesenchymal Stromal Cells.

Currently, the main drawback of ceramic scaffolds used in hard tissue regeneration is their low mechanical strength. Stabilized zirconia, especially the tetragonal 3% yttrium-stabilized zirconia (YSZ) phase, has been considered as a bioinert ceramic material with high mechanical strength. In the present work, flexible nanofibrous YSZ scaffolds were prepared by electrospinning. The obtained scaffolds showed remarkable flexibility at the macroscopic scale, while retaining their stiffness at the microscopic scale. The surface nanoroughness of the scaffolds could be tailored by varying the heat treatment method. Our results demonstrate that the osteogenic differentiation and mineralization of seeded human mesenchymal stromal cells were supported by the nanofibrous YSZ scaffolds, in contrast to the well-known bioinert behavior of bulk YSZ. These findings highlight that flexible ceramic scaffolds are an appealing alternative to the current brittle ceramics for bone tissue regeneration applications.

Highly stable, luminescent core-shell type methylammonium-octylammonium lead bromide layered perovskite nanoparticles.

A new protocol for the synthesis of a highly stable (over 2 months under ambient conditions) solution-processed core-shell type structure of mixed methylammonium-octylammonium lead bromide perovskite nanoparticles (5-12 nm), having spherical shape, color tunability in the blue to green spectral region (438-521 nm) and a high photoluminescence quantum yield (PLQY) of up to 92% is described. The color tunability, high PLQY and stability are due to the quantum confinement imparted by the crystal engineering associated with core-shell nanoparticle formation during growth.

Perovskite Materials for Light-Emitting Diodes and Lasers.

Organic-inorganic hybrid perovskites have cemented their position as an exceptional class of optoelectronic materials thanks to record photovoltaic efficiencies of 22.1%, as well as promising demonstrations of light-emitting diodes, lasers, and light-emitting transistors. Perovskite materials with photoluminescence quantum yields close to 100% and perovskite light-emitting diodes with external quantum efficiencies of 8% and current efficiencies of 43 cd A(-1) have been achieved. Although perovskite light-emitting devices are yet to become industrially relevant, in merely two years these devices have achieved the brightness and efficiencies that organic light-emitting diodes accomplished in two decades. Further advances will rely decisively on the multitude of compositional, structural variants that enable the formation of lower-dimensionality layered and three-dimensional perovskites, nanostructures, charge-transport materials, and device processing with architectural innovations. Here, the rapid advancements in perovskite light-emitting devices and lasers are reviewed. The key challenges in materials development, device fabrication, operational stability are addressed, and an outlook is presented that will address market viability of perovskite light-emitting devices.

Formation of nanocrystalline barium titanate in benzyl alcohol at room temperature.

Nanocrystalline barium titanate (8-10 nm crystallite size) was prepared at temperatures of 23-78 °C through reaction of a modified titanium alkoxide precursor in benzyl alcohol with barium hydroxide octahydrate. The room temperature formation of a perovskite phase from solution is associated with the use of benzyl alcohol as solvent medium. The formation mechanism was elucidated by studying the stability and interaction of each precursor with the solvent and with each other using various experimental characterization techniques. Density functional theory (DFT) computational models which agreed well with our experimental data could explain the formation of the solid phase. The stability of the Ti precursor was enhanced by steric hindrance exerted by phenylmethoxy ligands that originated from the benzyl alcohol solvent. Electron microscopy and X-ray diffraction indicated that the crystallite sizes were independent of the reaction temperature. Crystal growth was inhibited by the stabilizing phenylmethoxy groups present on the surface of the crystallites.

Concentration dependence on the shape and size of sol-gel-derived yttria-stabilized zirconia ceramic features by soft lithographic patterning.

Typical surface areas of 5 × 5 mm(2) were patterned with high-aspect-ratio micrometer- and submicrometer-sized structures of yttria-stabilized zirconia using a combination of micromolding in capillaries and sol-gel chemistry. The influence of precursor solution concentration and mold geometry on the final shape and dimensions of the patterned structures was investigated. At a precursor concentration of [Zr] = 0.724 mol/dm(3), isolated objects-due to the controlled cracking of patterned films-such as crosses (height 1.4 µm, width 6.0 µm) and "dog bones" (height 800-900 nm, width 900 nm) or patterned films (height 450 nm) were obtained, depending on the mold geometry. Lower precursor concentrations led to differently sized and shaped structures, with changes in dimensions of more than an order of magnitude. Employing a precursor concentration of [Zr] = 0.036 mol/dm(3) yielded isolated rings (height 100-150 nm, line width 20 nm) and squares (height 40 nm, line width 40 nm). A better understanding of the relationship between the precursor concentration, mold geometry, and observed coherent crack patterns in as-dried sol-gel structures may lead to new techniques in patterning isolated features.

Time-resolved small angle X-ray scattering study of sol-gel precursor solutions of lead zirconate titanate and zirconia.

The evolution of nanostructure in sol-gel derived lead zirconate titanate (PZT) and zirconia precursor sols at different hydrolysis ratios was investigated by small angle X-ray scattering (SAXS). The shape of the clusters in the zirconia sol could be described by the length-polydisperse cylindrical form factor. The zirconia-based clusters were characterized by a cross-sectional radius, r(0), of 0.28 nm and a practically monodisperse length of ca. 1.85 nm. These clusters were probably constructed of zirconia-related tetrameric building blocks. Similar cylindrical structural motifs were observed in PZT precursor sols with [H(2)O]/[Zr+Ti]=9.26 and 27.6, but the polydispersity in length was much higher. Clear scattering contributions from Ti and Pb centers were not detected, which was interpreted in terms of a homogeneous distribution of unbound lead ions in solution and the relatively low scattering intensity from any Ti-based clusters or oligomers that may have been present in the sols.

Development of nanoscale inhomogeneities during drying of sol-gel derived amorphous lead zirconate titanate precursor thin films.

The structural evolution of sol-gel derived lead zirconate titanate (PZT) precursor films during and after physical drying was investigated by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), selected area electron diffraction (SAED), and time-resolved X-ray diffraction (XRD). Films were deposited from initial 0.3 mol/dm(3) precursor sols with varying hydrolysis ratios. Zr-rich grains of 1-10 nm size, embedded in a Pb-, Zr-, and Ti-containing amorphous matrix were found in as-dried films. The Zr-rich regions were crystalline at hydrolysis ratios [H(2)O]/[PZT] < 27.6, and amorphous at ratios > 100. X-ray diffraction analysis of PZT and zirconia sols revealed that the crystalline nanoparticles in both sols are identical and are probably composed of nanosized zirconium oxoacetate-like clusters. This study demonstrates that time-resolved X-ray diffraction combined with electron energy loss spectroscopy mapping is a powerful tool to monitor the nanoscale structural evolution of sol-gel derived thin films.

Stacks of functional oxide thin films patterned by micromolding.

Stacks of up to five relief patterned functional oxide thin films were obtained by a low-cost and low-tech soft-lithographic patterning technique. Micromolding was used to pattern a film of a metal-organic precursor solution for Y-stabilized ZrO(2) (YSZO). Subsequent drying and pyrolysis yielded a line-patterned YSZO film. The process was repeated up to four times with a precursor solution for BaTiO(3) on top of the YSZO film, resulting in stacks of YSZO and BaTiO(3) lines with well-defined edges. This approach presents a step forward on the way to a versatile additive micropatterning technique with which simple multi-material device structures can be fabricated in a reliable, fast, and cost-effective manner.